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Having treated every other aspect of the skin, cosmetic formulators have recently discovered the cutaneous nervous system. The new category that has emerged as a result, neurocosmetics, can literally affect how the brain responds to topical treatment. The subject inaugurated the 2007 program of the New York Society of Cosmetic Chemists (NYSCC)1 and is certain to appear on many technical programs in the coming months.
Neurocosmetics targets nerve clusters sensitive to heat, cold, pain, itch and pressure. These receptors send signals through fibers in the skin to the spinal cord, which are then transmitted up to the cerebral cortex of the brain.
Physical coolants—ethanol is a common example—act through evaporation, and are a simple example of materials that create a nervous response. Lower-boiling compounds such as ether or acetaldehyde are even stronger coolants. However, evaporation as a mode of action does not qualify these materials as neuro-active.
Cosmetics have acted on neurons for many years—the cooling effect of menthol is a prime example. Menthol acts on thermoreceptors to provide the cool sensation via cold receptors, and can also create a hot or stinging pain sensation. Similarly, capsaicin can produce a hot sensation. Obviously, a great diversity of chemistry can trigger nerve impulses. A classic paper by Watson2 and his colleagues at Wilkerson Match explained the functioning of menthol and how to create a family of chemically related cooling compounds.
Watson attributed the cooling effect of menthol to a chemical action at the nerve ending associated with the sensation of cold. The necessary chemistry included a hydrogen-bonding group, compact hydrocarbon skeleton, correct HLB and molecular weight range of 150–350. These properties correspond to a classic drug receptor interaction.